PURPOSE: To evaluate the effects of a new microbubble contrast agent for ultrasound (US) on breast masses. MATERIALS AND METHODS: Thirty-four patients underwent color Doppler US before and after intravenous injection of a "contrast agent" containing microbubbles. The authors subjectively evaluated the increase in intensity of the Doppler signals, the changes in the vascular patterns, and the timing of the transit of the microbubble bolus. The diagnostic confidence was assessed before and after administration of contrast material. RESULTS: After contrast material injection, there was greater and longer signal enhancement in the cancers than in the benign lesions. The cancers displayed characteristic vascular morphologic features, with more additional vessels visualized in relation to the lesion and a greater increase in vascular tortuousity. Shunts between vessels were demonstrated in all cancers but were not seen in any benign lesion. The diagnostic confidence increased with use of the contrast agent. The appearance at contrast-enhanced US led to a change in the US diagnosis in four patients. This increased both sensitivity and specificity to 100%. CONCLUSION: Injection of a microbubble agent enabled accurate differentiation of benign masses from carcinomas.